This invention relates to abrasive compacts.
Abrasive compacts are used extensively in cutting, milling, grinding, drilling and other abrasive operations. Abrasive compacts consist of a mass of ultrahard particles, typically diamond or cubic boron nitride, bonded into a coherent, polycrystalline conglomerate. The abrasive particle content of abrasive compacts is high and there is generally an extensive amount of direct particle-to-particle bonding or contact. Abrasive compacts are generally sintered under elevated temperature and pressure conditions at which the abrasive particle, be it diamond or cubic boron nitride, is crystallographically or thermodynamically stable.
Some abrasive compacts may additionally have a second phase which contains a catalyst/solvent or binder material. In the case of polycrystalline diamond compacts, this second phase is typically a metal such as cobalt, nickel, iron or an alloy containing one or more such metals. In the case of PCBN compacts this binder material typically comprises various ceramic compounds.
Abrasive compacts tend to be brittle and in use they are frequently supported by being bonded to a cemented carbide substrate or support. Such supported abrasive compacts are known in the art as composite abrasive compacts. Composite abrasive compacts may be used as such in a working surface of an abrasive tool. The cutting surface or edge is typically defined by the surface of the ultrahard layer that is furthest removed from the cemented carbide support.
Examples of composite abrasive compacts can be found described in U.S. Pat. Nos. 3,745,623; 3,767,371 and 3,743,489.
Composite abrasive compacts are generally produced by placing the components necessary to form an abrasive compact, in particulate form, on a cemented carbide substrate. The composition of these components is typically manipulated in order to achieve a desired end structure. The components may, in addition to ultrahard particles, comprise solvent/catalyst powder, sintering or binder aid material. This unbonded assembly is placed in a reaction capsule which is then placed in the reaction zone of a conventional high pressure/high temperature apparatus. The contents of the reaction capsule are then subjected to suitable conditions of elevated temperature and pressure.
It is desirable to improve the abrasion resistance of the ultrahard abrasive layer as this allows the user to cut, drill or machine a greater amount of the workpiece without wear of the cutting element. This is typically achieved by manipulating variables such as average ultrahard particle grain size, overall binder content, ultrahard particle density and the like.
For example, it is well known in the art to increase the abrasion resistance of an ultrahard composite by reducing the overall grain size of the component ultrahard particles. Typically, however, as these materials are made more wear resistant they become more brittle or prone to fracture. Abrasive compacts designed for improved wear performance will therefore tend to have poor impact strength or reduced resistance to spalling. This trade-off between the properties of impact resistance and wear resistance makes designing optimised abrasive compact structures, particularly for demanding applications, inherently self-limiting.
Additionally, because finer grained structures will typically contain more solvent/catalyst or metal binder, they tend to exhibit reduced thermal stability when compared to coarser grained structures. This reduction in optimal behaviour for finer grained structures can cause substantial problems in practical application where the increased wear resistance is nonetheless required for optimal performance.
Prior art methods to solve this problem have typically involved attempting to achieve a compromise by combining the properties of both finer and coarser ultrahard particle grades in various manners within the ultrahard abrasive layer.
An approach to solving the problem of achieving an optimal marriage of properties between coarser- and finer-grained structures lies in the use of intimate powder mixtures of ultrahard grains of differing sizes. These are typically mixed as homogenously as possible prior to sintering the final compact. Both bimodal distributions (comprising two particle size fractions) and multimodal distributions (comprising three or more fractions) of ultrahard particles are known in the art.
U.S. Pat. No. 4,604,106 describes a composite polycrystalline diamond compact that comprises at least one layer of interspersed diamond crystals and pre-cemented carbide pieces which have been sintered together at ultra high pressures and temperatures. In one embodiment, a mixture of diamond particles is used, 65% of the particles being of the size 4 to 8 μm and 35% being of the size 0.5 to 1 μm. A specific problem with this solution is that the cobalt cemented carbide reduces the abrasion resistance of that portion of the ultrahard layer.
U.S. Pat. No. 4,636,253 teaches the use of a bimodal distribution to achieve an improved abrasive cutting element. Coarse diamond (larger than 3 μm in particle size) and fine diamond (smaller than 1 μm in particle size) is combined such that the coarse fraction comprises 60 to 90% of the ultrahard particle mass; and the fine fraction comprises the remainder. The coarse fraction may additionally have a trimodal distribution.
U.S. Pat. No. 5,011,514 describes a thermally stable diamond compact comprising a plurality of individually metal-coated diamond particles wherein the metal coatings between adjacent particles are bonded to each other forming a cemented matrix. Examples of the metal coating are carbide formers such as tungsten, tantalum and molybdenum. The individually metal-coated diamond particles are bonded under diamond synthesis temperature and pressure conditions. The patent further discloses mixing the metal-coated diamond particles with uncoated smaller sized diamond particles which lie in the interstices between the coated particles. The smaller particles are said to decrease the porosity and increase the diamond content of the compact. Examples of bimodal compacts (two different particle sizes), and trimodal compacts, (three different particles sizes), are described.
U.S. Pat. Nos. 5,468,268 and 5,505,748 describe the manufacture of ultrahard compacts from a mass comprising a mixture of ultrahard particle sizes. The use of this approach has the effect of widening or broadening of the size distribution of the particles allowing for closer packing and minimizing of binder pool formation, where a binder is present.
U.S. Pat. No. 5,855,996 describes a polycrystalline diamond compact which incorporates different sized diamond. Specifically, it describes mixing submicron sized diamond particles together with larger sized diamond particles in order to create a more densely packed compact.
U.S. Pat. Application No. 2004/0062928 further describes a method of manufacturing a polycrystalline diamond compact where the diamond particle mix comprises about 60 to 90% of a coarse fraction having an average particle size ranging from about 15 to 70 μm and a fine fraction having an average particle size of less than about one half of the average particle size of the coarse fraction. It is claimed that this blend results in an improved material behaviour.
The problem with this general approach is that whilst it is possible to improve the wear and impact resistances when compared with either the coarse or fine-grained fraction alone, these properties still tend to be compromised i.e. the blend has a reduced wear resistance when compared to the finer grained material alone and a reduced impact resistance when compared to the coarser grained fraction. Hence the result of using an intimate mixture of particle sizes is simply to achieve the property of the average intermediate particle size.
The development of an abrasive compact that can achieve improved properties of impact and fatigue resistance consistent with coarser grained materials, whilst still retaining the superior wear resistance of finer grained materials, is therefore highly desirable.